To conduct an ocean bottom seismic survey, a cable having a plurality of pairs of electrical conductors is laid out along a survey line and connected at one end to a recording device. At spaced locations along the cable, a "takeout" is provided that enables the leader wire of a string of geophones to be electrically connected to one of the pairs of conductors inside the cable. Each pair of conductors and the phones connected to it form a data channel so that acoustic waves that are reflected upward from underground geologic structures are recorded on that channel.
Once a set of data has been captured and recorded along the survey line, the cable is commonly dragged to the next survey line in preparation for another set of data. The cable may also be retrieved and redeployed in a new location to acquire another set of data.
In dragging or redeploying the cable from one survey line to the next, the seismic sensor package is commonly subjected to a number of potentially damaging obstacles on the ocean floor. In the past, the sensor, takeout, and the region of the cable in close proximity to them have been wrapped in heavy tape, and sometimes enclosed in a shrink-wrap plastic to minimize this damage. Unfortunately, this wrapping has often proved less than satisfactory, resulting in damage to the cable, the sensor, and often leading to catastrophic sea water in-leakage at the cable penetrations.
Aside from sealing the takeout, the tape and wrap applied to the cable are intended to anchor the sensor package along with its connecting leader cable. As the wrapping is damaged, the sensor package with the connecting leader cable is allowed to move freely, causing a further mechanical damage to the sensors and connecting cable.
In U.S. patent application Ser. No. 09/255,452, filed Feb. 19, 1999 and assigned to the same assignee as the present application, a combination takeout anchor and protective cover is disclosed. This application is incorporated herein by reference. This structure fills this long-felt need of protecting the sensor components on an ocean-bottom seismic cable.
The structure disclosed in that application very satisfactorily fulfills its intended purpose. However, the overall performance of the entire seismic cable could be improved if the anchor and protective cover were to more effectively couple the sensor components to the ocean bottom. More effective coupling of the sensor and ocean bottom leads to improved imaging of the geologic structures which may bear the desired hydrocarbons which the seismic survey is intended to find.
Thus, there remains a need for a coupling structure to firmly embed the geophones of an ocean-bottom seismic cable to the sea floor. Such a structure should also resist the bending force on the sensor elements as the cable is reeled aboard a host vessel, and should also help to protect the sensor package as the cable is dragged along the ocean floor.